Kabul, January 30 (RHC)-- Three U.S. security contractors have been killed in a shooting incident in the Afghan capital, Kabul, a U.S. official says. Speaking on condition of anonymity, the U.S. military official said that an “insider attack” was carried out at the Kabul airport on Thursday.

The official added that the victims were working for the U.S. Defense Department, without giving further information about the attack. Afghan officials said a fourth contractor was also wounded after an Afghan soldier opened fire at the contractors, adding that an Afghan national was also reported dead.

Scores of U.S.-led foreign troopers have died in the so-called "green-on-blue" or insider attacks, where Afghan soldiers turn their guns on foreign forces in Afghanistan, since the U.S. invasion of the Asian country in 2001.

The term "green-on-blue" comes from a color coding system used by the U.S. Army to refer to Afghan and foreign forces. U.S.-led troops have adopted special security measures in recent years to try to counter the threat of the so-called insider attacks.

The U.S. and its allies invaded Afghanistan in 2001 as part of Washington’s so-called war on terror. The offensive removed the Taliban from power, but insecurity continues across the country despite the presence of thousands of foreign troops.

The U.S.-led combat mission in Afghanistan ended on December 31, 2014. However, at least 13,500 foreign forces, mainly from the United States, have remained in Afghanistan in what the U.S. is calling a support mission.